This invention relates in general to methods of calibrating photometers and relates more particularly to a calibration method of utilizing optical fibers to transmit light signals to the photometer. The word "photometer" herein means any device, such as monochromators, spectrophotometers and interference filters, that separates an input beam into spatially separated spectral components for measurement of a spectral distribution.
In the figures, each element indicated by a reference numeral will be indicated by the same reference numeral in every figure in which that element appears. The first two digits of any 4 digit reference numerals and the first digit of any two or three digit reference numerals indicates the first figure in which its associated element is presented.
Spectrophotometers, monochromators and dispersion optics packages are readily available from a number of sources and can be used as general purpose measuring instruments and can also be incorporated into other instruments for special applications. For example, in plasma processing of wafers, process endpoint detection can be recognized by use of a monochromator or a spectrophotometer to detect the endpoint of a wafer processing step.
It is becoming increasingly important to be able to accurately detect the endpoint of various steps in a wafer fabrication process. Submicron linewidths and ultrathin layer thickness are becoming common, especially in state of the art devices. In such devices, overprocessing can undercut features, thereby severely affecting yield.
Also, these state of the art devices require processing in single wafer systems. To maintain the same throughput as conventional batch wafer process systems, these single wafer systems must complete each process step much faster than was required in the batch process systems. It is therefore important for system throughput to be able to stop a process step as soon as it is completed. Although it was acceptable to run a batch wafer process system for a preselected interval, in single wafer systems it is important to be able to accurately detect the endpoint of a process step so that processing can be quickly terminated.
In virtually all dry etch processes, such as plasma etch, reactive ion etch (RIE), ion milling, reactive ion beam etch (RIBE), and magnetron etching, light is emitted from the gas phase reactants, from the gas phase products and/or from the film being etched. Etch endpoint occurs when the exposed portion of a film being etched has been etched to an interface between that film and an underlayer.
At etch endpoint, some product species cease to be produced and some reactant species cease to be consumed. Therefore, in the gas phase, the reactant species quickly increase in concentration and the product species quickly decrease in concentration. These changes produce concomitant changes in the associated emission and/or absorption spectra intensities of the gas phase and film. Various endpoint detectors are designed to detect this change in optical intensity. Similar changes occur in the optical output spectra of other types of wafer processing systems.
In the endpoint detection system of FIG. 1, light is passed through a fiber optic cable 10 from a wafer processing system, such as the Applied Materials PE5000 plasma chamber 11, to a spectral detector such as monochromator system 12. This system includes a monochromator 13 that is adjusted to direct onto a photomultiplier tube 14 light of a wavelength that changes in intensity when an etch process step reaches an underlayer interface. Light from plasma chamber 11 passes through fiber optic cable 10, through an entrance slit 16, diffracts off of grating 15 and a portion of the diffracted light passes through an exit slit 17 to the photomultiplier tube. A high voltage power supply applies to the photomultiplier tube a voltage that can be adjusted to vary the gain of the photomultiplier tube.
A motor is connected by a drive shaft to a radiation dispersive element such as concave holographic grating 15. This enables the dispersive element to be rotated to change the wavelength component that is directed onto the photomultiplier tube. An endpoint detection system is responsive to the output of the photomultiplier tube to extract in real time a portion of this output signal that is indicative of the endpoint of a process step. In an etch process, this endpoint detection system detects when the etch process reaches and clears a layer interface in the wafer, thereby defining the end-of-etch condition.
A spectrophotometer can be utilized in place of monochromator 13 and photomultiplier tube 14 to provide to the endpoint detection system a spectral output. In such a system, the endpoint detection system would extract from this spectral output the amplitude of the spectral distribution of the chemical species that is being monitored for endpoint detection. Such a system has the advantage of utilizing more than one frequency component of light, thereby, via mathematical analysis that utilizes the increased amount of data, improving the signal to noise ratio of the light from the chemical component being monitored.
Because the spectral distribution of the light from the plasma chamber typically exhibits a number of sharp peaks characteristic of the chemical species in the plasma chamber, it is important that the photometer be accurately calibrated to assure accurate detection of these peaks. The conventionally available photometer modules are typically calibrated at the factory. These modules also commonly include some mechanism for recognizing when the holographic grating is rotated to a preselected reference orientation referred to as the "home location".
For example, in the monochromator module from Instruments SA, Inc., the holographic grating is rotated in response to rotation of a lead screw. Attached to the lead screw is a mechanical counter that counts the number of revolutions of the lead screw. This counter provides a visual indication of the approximate rotation of the lead screw. A stepper motor is utilized to turn the lead screw a controlled number of steps. An electronic record of the number of times the motor was stepped provides an electronic indication of the rotational orientation of the holographic grating. There are 400 steps per revolution and one complete revolution corresponds to a wavelengh change of 100 nm for the light incident on the exit slit. Thus, each step corresponds to a 0.25 nm change in wavelength at the exit slit.
Unfortunately, various causes, including mechanical wear and slippage, can result in miscalibration of the photometer. Because accurate calibration is required for efficient and accurate wafer processing and because high quality processing is required for commercially acceptable chip throughput, it is important to assure that the photometer calibration is correct at all times. Therefore, a mechanism and method are needed that ensure accurate ongoing calibration of the photometer. It is known that SC Technology incorporates a mercury lamp source for calibration of its endpoint detection system, but no details of these calibration processes are known.